Several other rule sets have systems that feel to me like they were built to satisfy similar goals. ACKS (2012) differentiates between repertoire, which are the spells available for casting, and formulae, which are all the other spells that a mage has access to (in something like a library). Mages in ACKS can swap spells in the repertoire with formulae, but only at extremely high cost (ACKS core, page 67):

An arcane spellcaster who already has a full repertoire of spells may sometimes wish to replace one spell in his spell repertoire with another of equal level. It costs 1 week of game time and 1,000gp for each spell level to replace a spell in the repertoire with another.

D&D 5E (2014) uses a similar approach where there is a difference between known and prepared spells but then adds an additional layer of complexity with spell slots, which are different in 5E than in previous editions, and function essentially as level-rated mana or spell points. To learn a new spell, 5E wizards must write the spell in a spellbook, which serves a similar function to formulae in ACKS, though there is only cost involved in the initial copying step, not when making the new spell available for preparation (5E Player’s Handbook, page 114):

For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it. Once you have spent this time and money, you can prepare the spell just like your other spells.

Abstractly, both systems represent two pools of spells, essentially online and offline. I find both systems somewhat cumbersome to use and difficult to explain.

The traditional approach, probably represented most paradigmatically in AD&D (1978), also uses spellbooks as collections of offline spells, along with a complicated host of intelligence-based limits and checks to learn new spells (AD&D Players Handbook, page 10):

Acquisition of Heretofore Unknown Spells: Although the magic-user must immediately cease checking to determine if spells are known after the first complete check of each spell in the level group, or immediately thereafter during successive checks when the minimum number of spells which can be known is reached, it is possible to acquire knowledge of additional spells previously unknown as long as this does not violate the maximum number of spells which can be known. New spells can be gained from captured or otherwise acquired spell books or from scrolls of magic spells. In the latter event the scroll is destroyed in learning and knowing the new spell or spells.

Actually following all the AD&D procedures results in nicely differentiated magic-users that can acquire new spells from adventuring, but the overhead is rather high and the various rules are scattered all over at least the Players Handbook. The AD&D approach also warmly embraces and rewards high-maintenance spreadsheet-assisted play, which is not what I am looking for.

Below is an approach I have been working on, encoded in two rules: Attune and Scribe. The rules are written in cryptic Hexagram style, but for D&D application, replace Magic rating with class level or your favorite determinant of spell capacity. The transaction cost of swapping spells between equipment slots and spell slots is represented by magic ink, the cost of which needs to be squared with the other relevant economies of gameplay.

(For anyone that does not catch the allusion, this approach is inspired by Dark Souls.)

Attune. To attune a Spell, consume a Spell Scroll and add the Spell from the Scroll to the list of Attuned Spells. Attune no more Spells than the Magic rating. For example, an Adventurer with Magic rated 3 may attune no more than three spells.

Scribe. To scribe a Spell Scroll, consume magic ink, add a Scroll of an Attuned Spell to the Gear list, and optionally remove the Spell from the list of Attuned Spells. Like all items, each Spell Scroll occupies one Gear slot.